1. Field of the Invention
The present invention relates to medical instruments generally and more particularly to a digital-type tone-pip-signal generator for an auditory-evoked-potential-measuring system.
2. Description of the Prior Art
Commonly, medical examination to identify and diagnose hearing defects includes observing the subjective response to auditory stimuli. The subjective sensitivity to stimuli level is measured at several auditory frequencies. Where such measurements are impractical, as, for example, of the very young, the evoked-action potentials resulting from the auditory stimulation may be measured.
Since the evoked-action potentials are often masked by other (EEG-type) brain-activity potentials, systems for measuring auditory-evoked potentials commonly employ a periodic stimulus. A periodic stimulus evokes a periodic potential which may be separated from the other more random potentials with a filter tuned to the periodic rate.
A stimulus commonly employed with auditory-evoked-potential measuring systems is that which is referred to as a tone pip. A tone pip includes a number of cycles of the auditory frequency the amplitude of which is defined by an envelope having an attack portion and a decay portion both chosen to minimize system ringing. A representative tone-pip signal is illustrated in FIG. 1 of the drawing generally designated by the number 10. The auditory cycles are designated by the number 12 and the attack and decay portions of the envelope are designated 14 and 16, respectively. (For clarity, the cycle period is shown expanded (distorted) with respect to the attack and decay periods.)
A prior-art-type generator for developing a tone-pip signal includes a linear-type sine-wave generator for developing a signal at the auditory frequency, a linear-type ramp generator for developing a signal representing the tone-pip envelope and a voltage-(gain) controlled amplifier for combining the signals to develop the tone-pip signal.
Unfortunately, it is difficult to develop a tone-pip signal of high accuracy with the above-mentioned prior-art-type generator. Additionally, improvements in signal accuracy are generally at the expense of circuit complexity.
SUMMARY OF THE PRESENT INVENTION
It is therefore a material object of the present invention to provide a generator for developing a highly accurate tone-pip signal.
Another object of the present invention is to provide a simple tone-pip-signal generator.
Briefly, a tone-pip-signal generator in accordance with the present invention includes a counter for counting cycles of a clock signal which occur following the time marked by a reset signal to develop signals representing the accumulated count and a gate circuit for coupling the clock signal to the counter until a predetermined count is accumulated. The generator also includes a memory storing numbers representing successive samples of the tone-pip signal, the memory being addressed by the counter developed signals to retrieve in succession the numbers representing the tone-pip signal. Additionally, the generator includes an exclusive-OR-gate array for inverting the signals developed by the memory to selectively invert the tone-pip signal and a digital-to-analog converter for developing the tone-pip signal from the memory developed signals.
An important advantage of the present invention is the accuracy of tone-pip signals which may be developed by generators in accordance therewith.
Other advantages of the present invention include the simplicity, economy and efficiency of generators in accordance therewith.
These and other objects and advantages of the present will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment illustrated in the drawing.